/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.atomic;

import java.io.Serializable;

/**
 * One or more variables that together maintain an initially zero
 * {@code long} sum.  When updates (method {@link #add}) are contended
 * across threads, the set of variables may grow dynamically to reduce
 * contention. Method {@link #sum} (or, equivalently, {@link
 * #longValue}) returns the current total combined across the
 * variables maintaining the sum.
 *
 * <p>This class is usually preferable to {@link AtomicLong} when
 * multiple threads update a common sum that is used for purposes such
 * as collecting statistics, not for fine-grained synchronization
 * control.  Under low update contention, the two classes have similar
 * characteristics. But under high contention, expected throughput of
 * this class is significantly higher, at the expense of higher space
 * consumption.
 *
 * <p>LongAdders can be used with a {@link
 * java.util.concurrent.ConcurrentHashMap} to maintain a scalable
 * frequency map (a form of histogram or multiset). For example, to
 * add a count to a {@code ConcurrentHashMap<String,LongAdder> freqs},
 * initializing if not already present, you can use {@code
 * freqs.computeIfAbsent(k -> new LongAdder()).increment();}
 *
 * <p>This class extends {@link Number}, but does <em>not</em> define
 * methods such as {@code equals}, {@code hashCode} and {@code
 * compareTo} because instances are expected to be mutated, and so are
 * not useful as collection keys.
 *
 * @author Doug Lea
 * @since 1.8
 */
public class LongAdder extends Striped64 implements Serializable {

  private static final long serialVersionUID = 7249069246863182397L;

  /**
   * Creates a new adder with initial sum of zero.
   */
  public LongAdder() {
  }

  /**
   * Adds the given value.
   *
   * @param x the value to add
   */
  public void add(long x) {
    Cell[] as;
    long b, v;
    int m;
    Cell a;
    if ((as = cells) != null || !casBase(b = base, b + x)) {
      boolean uncontended = true;
      if (as == null || (m = as.length - 1) < 0 ||
          (a = as[getProbe() & m]) == null ||
          !(uncontended = a.cas(v = a.value, v + x))) {
        longAccumulate(x, null, uncontended);
      }
    }
  }

  /**
   * Equivalent to {@code add(1)}.
   */
  public void increment() {
    add(1L);
  }

  /**
   * Equivalent to {@code add(-1)}.
   */
  public void decrement() {
    add(-1L);
  }

  /**
   * Returns the current sum.  The returned value is <em>NOT</em> an
   * atomic snapshot; invocation in the absence of concurrent
   * updates returns an accurate result, but concurrent updates that
   * occur while the sum is being calculated might not be
   * incorporated.
   *
   * @return the sum
   */
  public long sum() {
    Cell[] as = cells;
    Cell a;
    long sum = base;
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null) {
          sum += a.value;
        }
      }
    }
    return sum;
  }

  /**
   * Resets variables maintaining the sum to zero.  This method may
   * be a useful alternative to creating a new adder, but is only
   * effective if there are no concurrent updates.  Because this
   * method is intrinsically racy, it should only be used when it is
   * known that no threads are concurrently updating.
   */
  public void reset() {
    Cell[] as = cells;
    Cell a;
    base = 0L;
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null) {
          a.value = 0L;
        }
      }
    }
  }

  /**
   * Equivalent in effect to {@link #sum} followed by {@link
   * #reset}. This method may apply for example during quiescent
   * points between multithreaded computations.  If there are
   * updates concurrent with this method, the returned value is
   * <em>not</em> guaranteed to be the final value occurring before
   * the reset.
   *
   * @return the sum
   */
  public long sumThenReset() {
    Cell[] as = cells;
    Cell a;
    long sum = base;
    base = 0L;
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null) {
          sum += a.value;
          a.value = 0L;
        }
      }
    }
    return sum;
  }

  /**
   * Returns the String representation of the {@link #sum}.
   *
   * @return the String representation of the {@link #sum}
   */
  public String toString() {
    return Long.toString(sum());
  }

  /**
   * Equivalent to {@link #sum}.
   *
   * @return the sum
   */
  public long longValue() {
    return sum();
  }

  /**
   * Returns the {@link #sum} as an {@code int} after a narrowing
   * primitive conversion.
   */
  public int intValue() {
    return (int) sum();
  }

  /**
   * Returns the {@link #sum} as a {@code float}
   * after a widening primitive conversion.
   */
  public float floatValue() {
    return (float) sum();
  }

  /**
   * Returns the {@link #sum} as a {@code double} after a widening
   * primitive conversion.
   */
  public double doubleValue() {
    return (double) sum();
  }

  /**
   * Serialization proxy, used to avoid reference to the non-public
   * Striped64 superclass in serialized forms.
   *
   * @serial include
   */
  private static class SerializationProxy implements Serializable {

    private static final long serialVersionUID = 7249069246863182397L;

    /**
     * The current value returned by sum().
     *
     * @serial
     */
    private final long value;

    SerializationProxy(LongAdder a) {
      value = a.sum();
    }

    /**
     * Return a {@code LongAdder} object with initial state
     * held by this proxy.
     *
     * @return a {@code LongAdder} object with initial state held by this proxy.
     */
    private Object readResolve() {
      LongAdder a = new LongAdder();
      a.base = value;
      return a;
    }
  }

  /**
   * Returns a <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAdder.SerializationProxy">
   * SerializationProxy</a> representing the state of this instance.
   *
   * @return a {@link SerializationProxy} representing the state of this instance
   */
  private Object writeReplace() {
    return new SerializationProxy(this);
  }

  /**
   * @param s the stream
   * @throws java.io.InvalidObjectException always
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.InvalidObjectException {
    throw new java.io.InvalidObjectException("Proxy required");
  }

}
